In response to public criticism, a number of national and international agencies as well as airplane manufacturers and engine manufacturers are in the process of establishing more stringent noise emission standards for new type aircraft. Some airports, such as London Heathrow, Washington National and John Wayne, limit aircraft traffic based on noise emissions to the surrounding communities.
Typically current airport noise rules limit the amount of jet engine noise which can be emitted during takeoff and during approach to landing. Engine noise produced during takeoff is usually the highest contributor because the engine is at its highest power setting. Noise generated by an airplane jet engine has many components including fan noise, combustion noise, airframe noise and jet noise. Jet noise is caused by the shearing of three different airflows--the airflow from the engine fan duct exhaust with the cooler ambient airflow and the airflow from the engine core exhaust and the engine fan duct exhaust. It is similar to the noise one commonly hears from a high pressure air hose. Jet noise is the most prevalent engine noise component at high engine thrust conditions.
Historically, engine noise suppression has been achieved most dramatically by increasing secondary-to-primary mass flow bypass ratios from values near one, such as engines on the Boeing 727 and 737-200, to values nearer to five and up to eight such as on the Boeing 757, 767 and 777. The term "bypass ratio" refers to the ratio of (i) the mass flow rate of air which bypasses the engine core and is directed along a fan duct located between the engine core and the nacelle, to (ii) the mass flow rate of air which travels through the engine core. It has been found that increasing the bypass ratio decreases peak jet velocities, shear layer velocity gradients and turbulence thereby resulting in lower noise emissions.
Other attempts to reduce engine noise have been directed to various types of "hush kits" such as free mixers (i.e. ejectors) and forced mixers. The hush kit is typically connected to the aft end of the engine. It uses multiple lobes or spokes to mix the high velocity hot streams from the engine with the cooler lower velocity free streams of the surrounding air. The resulting mixing decreases peak jet velocity and shifts the noise from low frequency to high frequency where it can be more efficiently absorbed during atmospheric propagation.
Many of the above approaches have been successful in reducing engine noise levels. However, further reduction in jet engine noise is needed. Newer, higher bypass engines have yet to be fully developed, whereas hush kits tend to add complexity and weight to an already complex engine system.
A number of other conventional engine noise reduction systems using suction devices have been disclosed. For example, U.S. Pat. No. 3,095,696 by Rumble discloses a jet engine having an internal space of reduced pressure which surrounds the length of the engine for reducing engine noise emissions. Furthermore, U.S. Pat. No. 3,371,743 by Chanaud et al discloses a jet engine having an internal suction source at the exhaust outlet for reducing engine noise emissions.
Other noise reductions systems using suction devices include U.S. Pat. No. 3,820,628 by Hanson which discloses a jet engine having guide vanes and flow splitters which have openings in communication with a suction source so as to remove the boundary layer and the resulting wakes.
U.S. Pat. No. 5,060,471 by Torkelson discloses a jet engine having porous acoustical lining wherein the pressure at opposite surfaces of the lining can be varied so as to control the direction of air flow through the acoustical lining thereby varying the sound attenuating characteristics of the lining.
Suction devices have also been used to reduce aerodynamic drag. For example, U.S. Pat. No. 5,297,765 by Hughes et al discloses an engine nacelle having a porous surface at the forward end of the nacelle such that the porous surface is in communication with a suction source in order to reduce drag associated with the nacelle. In addition, U.S. Pat. Nos. 4,296,899 and 4,171,785, both by Isenberg, and which are assigned to the assignee of the present invention, disclose an aerodynamic surface such as a wing having a plurality of slots through which boundary layer air can be drawn in order to maintain laminar flow and reduce drag.
Other suction devices for reducing drag include U.S. Pat. No. 5,368,258 by Johnson et al, which discloses an engine nacelle wherein gaps in the skin of the nacelle are in communication with a suction source so as to reduce drag caused by the gaps. And, U.S. Pat. No. 4,993,663 by Lahti discloses an engine nacelle which incorporates a suction bleed system at the forward portion of the nacelle to promote laminar flow and reduce drag.
Furthermore, U.S. Pat. No. 4,258,889 by Hunt, which is assigned to the assignee of the present invention, discloses an insert for a suction slot in a wing to maintain laminar flow.